Within cosmology, there are two entirely independent pillars which can jointly drive this field towards precision: Astronomical observations of primordial element abundances and the detailed surveying of the cosmic microwave background. However, the comparatively large uncertainty stemming from the nuclear physics input is currently still hindering this effort, i.e. stemming from the 2H(p,γ)3He reaction. An accurate understanding of this reaction is required for precision data on primordial nucleosynthesis and an independent determination of the cosmological baryon density. Elsewhere, our Sun is an exceptional object to study stellar physics in general. While we are now able to measure solar neutrinos live on earth, there is a lack of knowledge regarding theoretical predictions of solar neutrino fluxes due to the limited precision (again) stemming from nuclear reactions, i.e. from the 3He(α,γ)7Be reaction. This thesis sheds light on these two nuclear reactions, which both limit our understanding of the universe. While the investigation of the 2H(p,γ)3He reaction will focus on the determination of its cross- section in the vicinity of the Gamow window for the Big Bang nucleosynthesis, the main aim for the 3He(α,γ)7Be reaction will be a measurement of its γ-ray angular distribution at astrophysically relevant energies. In addition, the installation of an ultra-low background counting setup will be reported which further enables the investigation of the physics of rare events. This is essential for modern nuclear astrophysics, but also relevant for double beta decay physics and the search for dark matter. The presented setup is now the most sensitive in Germany and among the most sensitive ones worldwide.

Abstract The work describes the design, construction and commissioning of the underground cosmic-ray experiment Experiment with MultiMuon Array (EMMA). The experiment is built into the Pyhäsalmi mine, in the town of Pyhäjärvi, Finland. The aim of EMMA is to determine the elemental composition of cosmic rays at an energy region around 4 PeV, the energy region called the ’knee’ region. This is achieved by measuring the lateral density distribution of high-energy muons originating from Extensive Air Showers (EAS). The design calculations for the EMMA experiment, which are based on the use of the parametrization of the lateral density distribution of muons, the method of shower reconstruction, and the energy and composition indicators, are presented. A strategy for reconstructing the composition of the cosmic rays is presented and it demonstrates the potential of applying unfolding techniques to the EMMA data. The effect of an array extension on the performance of EMMA is studied. The hardware used in the EMMA experiment is presented starting with an overview of the array and its detector stations. The EMMA array employs three different particle detectors, for which the main technical properties are given, and their use in the EMMA array is presented. A description of the infrastructure of the experiment is given and the rock overburden at the EMMA site at the depth of 80 metres is documented. The work contains the latest analysis of EAS data recorded by the tracking detectors of the experiment, which demonstrates that the experiment is taking data as planned and that the data are according to EAS physics expectations. Methods for event selection and tracking efficiency correction are presented, after which the analysis results of measured track multiplicity spectra are given. The shape of the recorded multiplicity spectrum indicates that the simplest model of a knee-like spectrum with a pure proton composition can not explain the data and that further analysis of the spectrum is required.